Livestock Farms Hike MRSA Risk for Neighbors

Action Points

Explain that a study in the Netherlands found that living in an area of high density of cows, pigs, or veal calves increased the likelihood of nasal carriage of livestock-associated methicillin-resistant Staphylococcus aureus (MRSA) even for individuals who did not work with the animals.

Note that the risk of carriage of livestock-associated MRSA compared with other MRSA strains was increased for those in animal dense regions even after adjusting for direct animal contact, living in a rural area, or probable source of MRSA.

People living near livestock farms are at increased risk of acquiring an antibiotic-resistant pathogen – even if they have no direct contact with the animals, researchers reported.

In a geographic analysis in the Netherlands, residents of animal-dense areas had a significantly increased chance of carrying livestock-associated methicillin-resistant Staphylococcus aureus (LA-MRSA) rather than other forms of the pathogen, according to Beth Feingold, PhD, of Johns Hopkins University, and colleagues.

The finding may have implications for MRSA surveillance in the U.S., where large pig farms, for instance, are more common than in the Netherlands, Feingold and colleagues argued in the November issue of Emerging Infectious Diseases.

"Controlling the spread of livestock-associated MRSA requires attention to community members in animal-dense regions who are otherwise unaffiliated with livestock farming," Feingold said in a statement.

MRSA was initially thought of as a healthcare-associated pathogen, the researchers noted, but in the past decade or so, most MRSA infections have taken place in the community.

Among those community strains, LA-MRSA has been detected in several countries, and in the Netherlands it now accounts for about 40% of MRSA cases, they added.

Several studies have found that LA-MRSA carriage – in the nose, for instance – is high in people who work with animals such as pigs or cows but fades quickly if the contact is interrupted.

A 2007 study in the Netherlands found, however, that simply living in a rural area increased the risk of acquiring LA-MRSA.

Feingold and colleagues used patient-level data from that study to test the notion that living in an area with high pig density might increase the risk of acquiring the pathogen.

The 27 case-patients in the study were the first in a cluster of people who tested positive for LA-MRSA in a national surveillance program, while the 60 controls were also index patients, but tested positive for other forms of MRSA.

As expected, Feingold and colleagues found that most of those who had direct contact with pigs and cows were LA-MRSA case-patients – 10 of 13 or 76.9% and 7 of 8 or 87.5%, respectively.

On the other hand, only 12 of 27 people who had no direct contact with livestock, or 44.4%, had LA-MRSA.

In univariate and multivariate logistic regression analyses, an increased density of livestock in a region raised the risk of carrying LA-MRSA rather than other forms the pathogen, they found.

Specifically, doubling the density of pigs, cattle, and veal calves per municipality increased the odds of LA-MRSA carriage over other types of MRSA by 24.7%, 76.9%, and 24.1%, respectively, after adjusting for direct animal contact, living in a rural area, and the probable source of MRSA, Feingold and colleagues reported.

"These results challenge us to understand how these exposures could be occurring," Feingold said.

The researchers noted that although pig farming, for example, is big business in the Netherlands, its scale "is greatly overshadowed by the density of pig-farming operations in the U.S."

Some 10.9% of U.S. pig farms have more than 2,000 animals; just 6.8% of farms in the Netherlands have that many, they noted.

Feingold and colleagues cautioned that the study was small and there is the possibility of recall bias concerning exposure to animals. Also, they noted, participants were the index patients in a cluster, so that risks of secondary transmission could not be evaluated.

The study had support from the Johns Hopkins Center for a Livable Future and the Pew Charitable Trusts.

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